Jim Baggott

Multiverse theories are fashionable among a relatively small number of theoretical physicists specialising in foundational problems in cosmology, particle physics and quantum mechanics. Over the last few decades these theories have leached into the public consciousness, as science’s answer to much that we can’t otherwise explain about the universe we inhabit, the elementary particles that we have discovered in it, and the reason for our own existence.

These theories are also extremely controversial, with some scientists publicly voicing the concerns of many by arguing that they are entirely metaphysical in nature: they are ‘beyond physics’, and not scientific. They argue that multiverse theories are inherently untestable and do not belong in a science founded on the relationships between theories and empirical facts. In truth, most practicing scientists simply roll their eyes and breathe a heavy sigh when they see yet more speculative nonsense from theorists who should really know better, before turning their attentions to more serious matters.

We all enjoy the occasional flight of fancy, especially if it encourages us to think more deeply about our place in the cosmos. But these flights are now prefaced with statements such as: ‘According to the latest scientific thinking …’ In their public pronouncements, the multiversalists often fail to disclose the distinction between the status of their speculations and that of well-established, empirically justified scientific theories such as relativity and quantum mechanics.

Consequently, the public is being misled to the belief that this stuff is accepted science, a situation that must surely be ultimately damaging to the public perception and acceptance of its authority. If leading scientists can claim that the multiverse exists without any supporting evidence, why then should people believe anything that scientists say? Climate change? A scientific-political conspiracy. Astrology? Sure, why not? Intelligent design? In an article published in Nature in late 2014, cosmologist George Ellis and astrophysicist Joe Silk called on the scientific community to defend the integrity of physics against this onslaught from untestable metaphysical speculation.

One casualty in the guerrilla war that is now being fought in the pages of popular science journals and blogs is context. A good example is afforded by Philip Ball’s recent post on Prospect magazine’s science blog. Ball reports on a paper published by a team of astrophysicists in the Monthly Notices of the Royal Astronomical Society.

The paper concerns the nature of the relationship between dark energy, the mysterious energy of ‘empty’ space responsible for the observed accelerating expansion of the universe, and the formation of stars and galaxies. The density of dark energy determines the size of the ‘cosmological constant’, the infamous fudge-factor that Einstein introduced into his equations in order to prevent the universe from collapsing in on itself. Dark energy acts as a peculiar kind of anti-gravity—expanding space, pushing everything apart and counteracting the effects of gravity, which tends to pull everything together. We have absolutely no idea what it is.

The relationship between the density of dark energy (or the size of the cosmological constant) and galaxy formation is of interest because it is cited as a prime example of ‘fine-tuning’ in the universe. The argument goes that if the cosmological constant was large and positive then the universe would be ‘open’: it would expand too quickly for stars and galaxies to form. In such a universe there would be no opportunity for the creation of third-generation stars with accompanying planetary systems. If the cosmological constant was large and negative then the universe would be ‘closed’, collapsing in on itself long before life on any warm, wet, rocky planet has the opportunity to get started.

It seems that the cosmological constant characteristic of our universe has just the right small, positive, ‘Goldilocks’ value to ensure that life gets a chance.

But just how ‘special’ is this relationship? The paper reports the results of sophisticated computer simulations that suggest the relationship is not so special, after all. The physicists discovered that star formation is actually much less dependent on the size of the cosmological constant than might have been assumed, due to feedback from the formation of black holes at later times in the evolution of the universe.

This is interesting stuff, but of course what makes it blogworthy is that magic word ‘multiverse’. To explain away the fine-tuning problem, some theorists have speculated that ours is but one of a large (perhaps infinite) number of universes all with different cosmological constants. They then invoke some marvellously circular reasoning called the anthropic cosmological principle, which essentially says that we shouldn’t be at all surprised to find ourselves in a universe with a ‘Goldilocks’ cosmological constant, because otherwise we wouldn’t be here.

And this is where we lose the context. This paper isn’t about the multiverse, in the sense of promoting or testing ‘multiverse theory’. It’s about the relationship between the cosmological constant and galaxy formation. Yes, computer simulations are run for different values of the cosmological constant, and hence different ‘universes’, but this is done without assuming that these ‘universes’ are all equally real. In fact, the word multiverse appears just once, as a logical consequence of anthropic reasoning, in a single paragraph in the main body of the paper. There’s no mention of the multiverse in either the abstract or the discussion and conclusions section.

Yet Ball’s post is titled: ‘Just how special is human existence? The answer could lie in multiverse theory’. This is thoroughly misleading. As the title is reproduced in all links to the post on social media platforms, the deception is compounded. On publication, Prospect magazine tweeted a link with the caption: ‘New “multiverse” calculations shed light on one of science’s great puzzles.’ Yes, “multiverse” is in quotes, but ‘multiverse calculations’ again implies that this is all about a real scientific multiverse theory, with the capacity to perform calculations (and, by assumption, make predictions) when it’s actually not. Of course, Prospect is principally interested in getting your attention and, sadly, the word ‘multiverse’ is much more likely to do this than anything actually relevant to the paper in question.

Ball is a highly respected science writer and, of course, those who trouble themselves to read his post in full will find an accurate report of the contents of the paper—albeit with more discussion of the multiverse than might appear to be warranted. In fact, these calculations serve to undermine the conclusions of a famous (or infamous) article published in 1987 by Nobel laureate Steven Weinberg, in which he used anthropic arguments to place an upper bound on the cosmological constant, a decade before observations confirmed its small, positive value. In their paper, Salcido et al., write that their results bring ‘… into question whether the [fine-tuning] problem (the comparable energy densities of matter and dark energy) can be explained by an anthropic argument: the existence of dark energy (at the observed value) has negligible impact on the existence of observers or the ability of humanity to observe the cosmos.’ The italics are mine. Ball concludes his blog post with the observation: ‘The researchers say that we might have to think about the possibility that some other, unknown physical law limits the amount of dark energy—or perhaps that the explanation for it is different altogether.’

Far from suggesting that the ‘answer could lie in multiverse theory’, the paper actually exposes the fragility and superficiality of the arguments from which the idea of the multiverse is derived. By taking ‘multiverse theory’ out of context like this, the absurd notion of the multiverse as a valid scientific theory with the same status as quantum theory or big bang cosmology becomes normalised. It’s hard enough to resist the tide of metaphysical nonsense advertised as science when this is coming from the scientists themselves. Is it too much to ask that science writers and publishers resist the temptation to leap on this bandwagon at every opportunity?